1. Field of Invention
My invention relates to the field of optics, and more particularly to real time processing of optical signals with light.
2. Brief Description of the Prior Art
Real time optical signal processing technology began to develop at least as early as 1946, with processors capable of forming Hilbert transform pairs and limited voice processing. Processors using acoustooptic modulators date back to at least 1948, as illustrated by U.S. Pat. No. 2,451,465 to Barney. Since that time, interest in optical signal processing has steadily increased, despite the appearance and maturing of digital technology.
Though digital processors offer flexibility presently not possible in optics, interest continues to be drawn to optical solutions for a variety of reasons. The foremost of these is input bandwidth. While digital operation is difficult above bandwidths of a few tens of megahertz and nearly impossible above one hundred megahertz, acoustooptic processors routinely process bandwidths from the low megahertz region to a gigahertz. Optical signal processors operate exceptionally fast, with outputs available milliseconds to less than a microsecond after the input process is complete. The parallelism achieved in optics is also advantageous. It is not uncommon to perform operations on more than 10.sup.5 data elements, and with no more components than it takes to operate on a few. Still another feature of optics is that many desirable operations naturally lend themselves to easy implementation. The most important of these is the Fourier transform, which occurs in an almost trivial way in the optical domain. Multiplication is performed trivially, as well. Since optical systems usually have few components, they are often superior to their digital counterparts in size, power consumption, cost, and reliability.
The triple product processor is probably the most versatile of optical signal processors. Accepting three functions of time, f(t), g(t), and h(t), as inputs, the processor produces the ensemble of outputs ##EQU1## where T is an interval, and S is typically a rectangle. With appropriate choice of inputs, including both the signals to be processed and auxiliary functions, a variety of processing operations may be achieved. Illustrative examples may be found in P. Kellman, "Time Integrating Optical Signal Processing," doctoral dissertation, Stanford University, Stanford, Calif., June, 1970. Such operations include the ambiguity function and the raster-format transform.
Triple product processors employ acoustooptic modulators to achieve the required delays and use time-integrating detector arrays to generate the integrals. The first triple product processor (TPP), devised by Turpin (U.S. Pat. No. 4,225,938) for ambiguity function processing, was configured as a Mach-Zehnder interferometer. Each interferometer arm contained a pair of Bragg cells such that one arm comntributed the modulation f.sub.1 (t) g(t-x) to the output plane, while the other contributed f.sub.2 (t) h(t-y). The resulting interference term at the detector contained the desired product f.sub.1 (t) f.sub.2 (t) g(t-x) h(t-y). It was also recognized that two of the modulators could be combined and driven by the product f(t)=f.sub.1 (t)f.sub.2 (t), and that all of the modulators could be placed in one arm if desired. The drawback of such constructions was obvious: the large interferometer was extremely sensitive to relative movement of the components due to vibration and temperature changes and to thermal gradients in the air.
An incoherent-light TPP which avoided the sensitivity of the interferometer has been demonstrated. One of the many variants of the incoherent-light triple product processor found in the prior art is pictured in FIG. 1. In this scheme, light from a (coherent or incoherent) source 10 modulated by f(t).gtoreq.0 passes through two Bragg cells 14 and 15 oriented orthogonally and driven by g(t) and h(t), respectively. Lenses 17 and 19 serve to identify horizontal position on the detector array 20 with position along cell 14, and lenses 22 and 24 map delay in cell 15 to vertical position on the array. As a result, the intensity seen at the detector is EQU I(x,y,t)=f(t)g.sup.2 (t-x)h.sup.2 (t-y),
where the units for x and y are chosen so that the acoustic velocity is taken as unity. The detector array 20 integrates the intensity of the impinging light over an interval T and produces the outputs ##EQU2## where S now represents the locations of the detector elements. For some applications, this incoherent correlation may be sufficient. In case the coherent integration is required, Kellman, supra has shown how to obtain outputs of the form ##EQU3## by introducing reference "tones" with the inputs and detecting the output appropriately.
The Kellman architecture possesses a bandwidth limitation not present in the coherent-light architecture described above. This limitation can be seen by making the following observations: The Fourier transform of the light diffracted by cell 14 appears in the plane of cell 15 as a function of the horizontal space coordinate. All of this light must fall within the acoustic beam of cell 15 to be diffracted. Thus, the extent of the transform must be less than the acoustic beam height in cell 15. As the input bandwidth is increased, this becomes impossible to satisfy: larger bandwidth demands a smaller transducer capacitance and a corresponding decrease in beam height. At optical wavelengths, an insufficient aperture is provided to accommodate the full time-bandwidth product available in cell 14 if cell 15 is designed for too large a bandwidth. A pratical bandwidth limit on the order of 150 l MHz results.
If one wishes to obtain high bandwidth without returning to the instability of the Mach-Zehnder implementation, another architecture is required. One such architecture uses a construction similar to the incoherent-light TPP. Said architecture is described in M. A. Krainak and D. E. Brown, "Interferometric Triple Product Processor (Almost Common Path)," Applied Optics, 24, No. 9, May 1, 1985, pp. 1385-1388. The light source is replaced by a coherent source modulated by f(t). Bragg cell 15 is translated horizontally so that the light not diffracted by the first AO cell 14 is used to illuminate the second cell, with the diffracted light passing next to it. When, on the detector, the light diffracted by the second cell meets the light diffracted by the first, an interference term is produced which bears the desired product of g(t) and h(t). Since the two diffracted beams arrive from different angles, the desired result occurs on a spatial carrier of high frequency, which is stripped by passing through a holographic grating placed in the detector plane.
This almost-common-path design allows high bandwidth but suffers some practical problems. The high spatial frequency of the hologram imparts stringent mechanical stability requirements to the entire optical system. To block the undiffracted light from cell 15, a long optical path is required to form a plane where this light is disjoint from the desired contributions. The long path increases the stability problem. Also, because both the diffracted and undiffracted light from Bragg cell 14 must be carried through the system, the lenses must accommodate a larger range of angles and have larger apertures. In a system where geometric fidelity is critical, this stresses lens design.